1. Field of the Invention
The present invention relates to a device for recirculation of an exhaust gas in a series-mounted two-stage turbocharged internal combustion engine.
2. Description of the Related Art
In a known series-mounted two-stage turbocharged internal combustion engine, a large volume and a small volume turbocharger are arranged in series along the direction of the flow of gas (see Japanese Unexamined Patent Publication No. 50-129815). Such a two-stage turbocharger system is used for obtaining a wide range turbocharging operation, from a low engine speed area to a high engine speed area. Namely, when the engine speed is low, a turbocharging operation is obtained by a turbocharger having a small volume, and when the engine speed is high, a turbocharging operation is obtained by a turbocharger having a large volume. An exhaust bypass passageway is provided in the small-volume turbocharger, to bypass a turbine thereof, and an exhaust switching valve is arranged in the bypass passageway. The exhaust switching valve closes the bypass passageway when the engine is in a low speed condition, at which the large-volume turbocharger is not yet fully activated, so that all of the flow of the exhaust gas from the engine is introduced into the small-volume turbocharger, to obtain a turbocharging operation by the small-volume turbocharger. The exhaust switching valve opens the bypass passageway when the engine is in a high speed condition, at which the large-volume turbocharger is fully activated, and thus the flow of the exhaust gas bypasses the small-volume turbocharger and the turbocharging effect is obtained only from the large-volume turbocharger.
Also known is an exhaust gas recirculation system for reducing the amount of nitrogen oxide emissions from the gases exhausted from an internal combustion engine, with a single turbocharger, where an amount of exhaust gas is taken out from an engine exhaust line at a location upstream of the turbine of the turbocharger, and introduced into an engine intake line (see Japanese Unexamined Utility Model Publication No. 61-151063).
When carrying out an exhaust gas recirculation (EGR) operation in an internal combustion engine provided with a two-stage sequential turbocharger system, as in the prior art for an internal combustion engine provided with single turbocharger, the exhaust gas to be recirculated also can be taken from the exhaust line at a position upstream of the turbochargers which are arranged in series. In this case, however, the parts of the exhaust gas recirculating system, which are less resistant to high temperatures, such as a diaphragm made of rubber of an exhaust gas recirculating valve, are often thermally damaged after a relatively short use, because the EGR system is subjected to the high temperature of the exhaust gas in the two-stage turbocharger system. To avoid this thermal damage to parts of EGR system, the exhaust gas for the EGR operation may be taken out from the engine exhaust line at a position downstream of the turbines of the series of the turbochargers, where the temperature of the exhaust gas is lower, but a substantial drop in the pressure of the exhaust gas occurs at such a position, and therefore, the pressure differential between the pressure at the position where the exhaust gas is taken out and the position where the exhaust gas is introduced into the intake line becomes small, and this makes it difficult to obtain a desired amount of exhaust gas to be recirculated.